The invention is used, for example, in pressurised water reactors.
In conventional manner, the core of such a reactor is charged with nuclear fuel assemblies.
Each assembly comprises a bundle of nuclear fuel rods, the rods comprising a cladding which contains nuclear fuel pellets.
It may be advantageous, particularly in countries such as France where 80% of electricity is produced by nuclear reactors, for the overall power supplied by the reactors to vary in order to adapt to the requirements of the electrical power network which they supply.
In particular, it is desirable to be able to operate reactors with reduced overall power for a long period of time when the demand on the network is low, before returning if necessary to nominal overall power.
Nonetheless, operating each reactor in this manner, which allows the capacities thereof to be better exploited, must not involve safety problems.
One of the phenomena limiting the operability of nuclear rectors is in particular the phenomenon of Pellet/Cladding Interaction (PCI).
When the reactor operates at the nominal overall power PN thereof, the nuclear fuel rods are, according to the term used in the art, processed.
For a specific rod, the processing is characterised substantially by the radial clearance being closed between the pellets and the cladding, owing to the creep of the cladding and the swelling of the pellets.
Although there is no risk of fracture of the cladding during permanent operation owing to the thermomechanical equilibrium in the cladding at relatively low stress levels, a risk does arise as soon as the power provided by the rod in question varies significantly.
An increase of local power brings about an increase of the temperature in the rod. Given the difference of the mechanical characteristics (thermal expansion coefficient, Young's modulus) and the temperature difference between the pellet based on uranium oxide and the cladding which is conventionally of zirconium alloy, the pellet will expand more than the cladding and impose its deformation on the cladding.
Furthermore, the presence in the space between the cladding and the pellet of corrosive fission products, such as iodine, creates corrosion conditions under stress. In this manner, the deformation imposed by the pellet on the cladding during a transient occurrence of overall power may bring about a fracture of the cladding.
Such a fracture of the cladding is not admissible for safety reasons since it could result in fission products being released into the coolant system of the nuclear reactor.
The patent application EP-1 556 870 describes a method which, using the phenomenon of PCI, allows the limit values of the operating parameters of a nuclear reactor to be determined. More precisely, the limit values determined are such that, in the event of an accidental transient occurrence of overall power which will become evident with an increase in the local power in the entire core, the phenomenon of PCI will not result in a fracture of the nuclear fuel rod cladding.
This method thus allows the fields of use to be defined in which the nuclear reactor can operate in a safe manner, even in the event of an accidental transient occurrence of overall power. Alarms can also be introduced to verify that the limit values determined are not exceeded during the operation of the nuclear reactor.
The PCI phenomenon is particularly disadvantageous with respect to a specific operating method of nuclear reactors. This is Extended Reduced Power Operation (ERPO).
In France, extended reduced power operation is more precisely defined as being the permanent operation of the reactor, at an overall power PI less than or equal to, for example, approximately 92% of the nominal power PN thereof, for example, over a cumulative period of time of more than 8 hours in a given 24 hour period.
Such a method of operation has the effect of de-processing the rods.
During a reduction of the overall power, the power decreases locally. There is consequently a temperature reduction in the pellets and in the cladding of each rod, which brings about a reduction of the thermal expansions of these elements. Since each pellet has a greater thermal expansion coefficient than that of the associated cladding, it therefore retrocedes a greater absolute expansion.
This is further amplified by the fact that, for a specific local power reduction, the temperature variation in each pellet is greater than that in the cladding.
In this manner, during operation in ERPO mode, for the rods in which the contact between the cladding and the pellets is not established, the radial clearance increases. With regard to the rods in which the clearance was closed, the clearance re-opens.
In the event of reopening of the clearance, there is creeping in terms of compression towards the inner side of the cladding owing to the effect of pressure. The stresses which appear in the cladding in the event of an accidental transient occurrence of power during operation in ERPO mode thus reach greater values than if the transient occurrence takes place when the reactor is operating at nominal overall power.
The risks of a fracture owing to the PCI phenomenon are therefore increased when the reactor operates in ERPO mode.
In order to allow nuclear reactor operators to evaluate the extent to which they are able to use ERPO mode, without compromising the integrity of the claddings of the rods, a parameter has been developed, the credit K.
This parameter which is representative of the operability of the nuclear reactor is defined by the formula:
  K  =            K      0        -                  ∑        i            ⁢                        A          i                ⁢                  T          i                      +                  ∑        j            ⁢                        B          j                ⁢                  T          j                    
where K0 is the initial value of the credit K;
Ai is a deprocessing coefficient calculated from the laws of deprocessing;
Ti is the duration of a phase i of use of the ERPO mode;
Bj is a reprocessing coefficient calculated from the reprocessing laws; and
Ti is the duration of a phase j of nominal overall power operation after a period of operation in ERPO mode.
The operator is capable using this formula of calculating the development during a cycle of the value of the credit K in accordance with the successive phases of operation in ERPO mode and at nominal overall power.
The lower the value of the credit K is, the less possibility there is for the operator to use ERPO mode. When the value of the credit K is 0, the operator can no longer function in ERPO mode and must only operate the reactor at nominal power or shut it down.
In order to increase the value of the credit K, the operator may choose to operate the reactor at nominal overall power for a specific length of time.
The establishment of this formula, and in particular that of the coefficients Ai and Bj which takes almost two years, requires very significant calculations which are carried out over a period of several months on processors operating in parallel.
Taking into account the complexity involved in calculating the coefficients Ai and Bj, the determination of the value of the credit K is carried out in a generic manner for a specific reactor, fuel assembly and control which requires the introduction of a number of careful considerations.
Although the use of the credit K allows safe operation to be ensured for the nuclear reactor, unfortunately it therefore leads to limited operability.